As the driver slams on his breaks, passengers in the back of the mini bus lurch forwards. The perilous pedestrian blocking our path, however, doesn't seem to be in any hurry to cross the road. Hardly surprising since, even at top speed, this three-toed sloth can only muster 0.15mph.

With his unkempt moss-matted hair, long, curly talons and a permanently fixed grin, he looks like a mad old man with hygiene problems. Yet there's also something inexplicably appealing about him.

Preferring to spend most of their time in the branches of cycropia trees, sloths only come down to the ground once a week to release their bowels. So our chance collision en route to the natural pools at Rio Celeste is quite an occasion.

The subject of cult websites, Twitter trends and even kept as pets by A-list celebrities, these verging-on-ugly creatures are Costa Rica's most famous animal residents. But they are only one of the many species that inhabit this small Central American country, which, although occupying just 0.03% of the planet's surface, accounts for 5% of the world's biodiversity.

I, like many other tourists, have come here to see these animals in their natural environment, but a visit to the Jaguar Research Centre in Puerto Viejo on the Caribbean coast is a good place to get to grips with what's on offer.

There are approximately 100 rescue centres in Costa Rica, taking care of injured animals and staffed mainly by volunteers. Although it's name would suggest otherwise, there aren't any jaguars at this particular one, but I do see monkeys, snakes, ocelots and, of course, sloths. Many, sadly, are the victims of man-made accidents; a spectacled owl, who lost an eye after flying into a car's headlamps, can no longer hunt and is now a permanent resident.

Puh, a volunteer from England, is cradling a two-toed sloth wrapped in a blanket. Pointing to the creature's sharp claws, she says: "This one's a girl but we've named her Tyson... because she always likes to fight."

Puh tells me she originally came to Puerto Viejo in the Eighties and loved it, but when she returned four years ago, she discovered the one-time hippy hangout had deteriorated into a party town for gap year travellers. She was about to leave when she stumbled upon the Jaguar Research Centre.

"I phoned up for the opening times and I already knew I was going to stay," she says, smiling. She's been back every year since, working solidly in an office at home for five months to save enough money to spend the rest of her time here, caring for animals.

Walking through the sandy streets of Puerto Viejo, where travellers desperately try to pedal their wares and drunken kids set off fireworks at night, I have to agree with Puh that the town is utterly charmless. Fortunately, there are still people willing to stick around and celebrate the wonderful wildlife on its doorstep.

Ricky, a 64-year-old Rasta man, who guides trails through the Manzanillo National Wildlife Refuge, believes he has a responsibility to protect nature for future generations. His knowledge, gleaned from his grandma, an indigenous Bribri, has greater roots in cultural folklore than scientific research, but his tours are still entertaining and his love of nature is highly applaudable.

As we hike along the beach through mangroves and into the forest, he blows kisses to poisonous eyelash viper snakes curled up in trees, and demonstrates his love for golden war spiders by allowing one to climb across his face. I'm slightly sceptical, though, about his theory that a bite from a bullet ant can cure arthritis.

Although the government in Costa Rica has implemented laws to protect the environment, many people refuse to observe them, and it's passionate custodians, such as Ricky and Puh, who are really working hard to protect the country's greatest assets.

Thanks to the growth in eco-tourism, there's also an economic benefit to their efforts.

Once a simple farmstead, the Magsaysay Lodge in the Sarapiqui rainforest is now a guest house for tourists. Bordering the Braulio Carrillo National Park, there are more varieties of plants, birds and butterflies found here than in all of Europe.

I arrive on a tractor, plunging through shallow rivers and climbing into the hills. A pair of Chestnut-mandibled toucans swoop past, their guttural, grating calls competing with the undulating synths of an oropendola bird.

The sweet smell of caramelised, fried plantain leads me to the lodge, where hummingbirds dart across the garden. After dark, the owners light a bonfire, and we roast marshmallows while listening to a lecture from a local bat researcher, who's set up nets to catch some of the 100 bat varieties that come here.

A nearby coffee co-operative, in the village of San Miguel de Sarapiqui, is also reaping the financial rewards of eco-tourism. Unlike other Central American countries, every coffee plantation in Costa Rica is owned by a co-operative, meaning funds are fairly distributed. Many supplement their incomes by offering tours of their sites.

The soil here is extremely fertile, thanks mainly to volcanic activity in the region. The best known peak is the Arenal volcano, and the town of La Fortuna - which has grown up around its base - has become a hub for adventure tourism.

I climb to the top of Arenal on an unfortunately rainy day. A thick mist swirls through the forest, where the muddy ground has turned to soup, and buttress roots protrude from the earth like veins in a weightlifter's arm, providing ample seclusion for Costa Rica's rainbow collection of frogs.

Bio-diverse landscapes vary wildly across the country, although only 25% of the land is protected. On the Caribbean side, Tortuguero National Park is an important nesting site for turtles, while inland waterways offer a chance to spot iguanas, anteaters and Jesus Christ lizards, so named because they appear to walk on water.

A lot of land on the Pacific coast was destroyed during the Eighties McDonald's boom, when forest was felled for cattle grazing, but the Manuel Antonio Park, with its idyllic castaway beaches, is one of Costa Rica's most popular wildlife havens.

It's not often you have the chance to simultaneously sunbathe and spot animals, but during a stroll along the beach, I encounter iguanas basking on rocks, sloths dozing in trees, and capuchin monkeys jumping through the forest canopy.

Sadly, I also see raccoons foraging for leftover sandwiches and packets of crisps in bins, a sign that the animal world is not immune to the impact of human encroachment. The sprawling hotel developments leading up to the park's gates also undermine any respect for the national park's status.

By 2021, the government hopes Costa Rica will become a 0% pollution country, an ambitious goal. With neighbours Nicaragua and Panama having openly declared their intentions to industrialise at all costs, there's also a risk doors to Central America's biological corridor will be blocked off.

It may be one of the few countries in the world not to have an army, but if Costa Rica wants to protect its natural treasures and win a war against environmental change, they could have a fight on their hands.

:: G Adventure (; 0844 272 2040) offers a 16-day Costa Rica Adventure trip, covering Puerto Viejo, Tortuguero, Sarapiqui, Monteverde, Manuel Antonio and San Jose, from £999pp. Price includes accommodation, transport, some meals and activities. Flights extra.